adds support for the trailing commas in the enum-list rule

Here is the test:

```

$ cat enum-{commas,no-comma}.c
typedef enum {
  Little,
  Medium,
  Big,
} Size;
typedef enum {
  Little,
  Medium,
  Big
} Size;

$ ./printc/printc enum-{commas,no-comma}.c

typedef enum  {
        Little,
        Medium,
        Big
} Size;

typedef enum  {
        Little,
        Medium,
        Big
} Size;
```

frontc/cparser.mly

@@ -829,6 +829,8 @@ STRUCT type_name
     {ENUM ($2, [])}
   |		ENUM LBRACE enum_list RBRACE
     {ENUM ("", List.rev $3)}
+  |		ENUM LBRACE enum_list COMMA RBRACE
+    {ENUM ("", List.rev $3)}
   |		ENUM type_name LBRACE enum_list RBRACE
     {ENUM ($2, List.rev $4)}
 ;